1. Field of the Invention
The present invention relates to solid-state imaging devices and, more particularly, to a highly integrated solid-state charge-transfer type image sensor having an increased number of picture elements or "pixels" arrayed at a high density.
2. Description of the Related Art
In recent years, the number of pixels of solid-state image sensors as image input means has been increased in their high-resolution image processing performance. When the number of pixels on a substrate having a limited size is increased, each pixel is naturally micro-fabricated in size. The micro-fabrication of pixels strongly requires improvements in transfer efficiencies in horizontal and/or vertical charge transfer channel sections which transfer signal charge carriers between the pixels.
In a presently available solid-state image sensor, transfer electrodes are generally formed of a polycrystalline silicon thin film which is generally inferior in reliability, and an insulating interlayer for the electrodes is formed of a normal thermal oxide silicon layer. Therefore, if a transfer electrode is made thin and narrow in accordance with the micro-fabrication of pixels, an unwanted electric short-circuit or an undesirable potential distribution variation occurs in the transfer electrodes having poor film quality. In a charge-coupled image sensor having a single-layered transfer electrode structure, for example, a potential barrier and/or a potential pocket are/is formed in a region of charge transfer channel sections located immediately below gap sections between transfer electrodes. Such a phenomenon conspicuously reduces a transfer efficiency of signal charge carriers.
In order to solve the above problems, highly reliable new materials may be used for the transfer electrodes and the insulating interlayer. The use of such a new material, however, naturally requires a new manufacturing process. This requirement makes it difficult to manufacture image sensors and at the same time increases manufacturing cost. The difficulty in manufacture and the increase in cost are not desirable for manufacturers of semiconductors.